Synergistic inhibition effect of red tetrazolium and uracil on the corrosion of cold rolled steel in H3PO4 solution: Weight loss, electrochemical, and AFM approaches

Materials Chemistry and Physics 115 (2009) 815–824

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Synergistic inhibition effect of red tetrazolium and uracil on the corrosion of cold rolled steel in H3 PO4 solution: Weight loss, electrochemical, and AFM approaches Xianghong Li a,∗ , Shuduan Deng b , Hui Fu a a b

Department of Fundamental Courses, Southwest Forestry University, Kunming 650224, PR China Department of Wood Science and Technology, Southwest Forestry University, Kunming 650224, PR China

a r t i c l e

i n f o

Article history: Received 24 June 2008 Received in revised form 25 December 2008 Accepted 19 February 2009 Keywords: Metals AFM Adsorption Corrosion

a b s t r a c t The synergistic inhibition effect of red tetrazolium (RT) and uracil (Ur) on the corrosion of cold rolled steel (CRS) in 1.0–10.0 M H3 PO4 solution was first studied by weight loss and potentiodynamic polarization methods. Atomic force microscope (AFM) provided the CRS surface conditions. The results revealed that RT had a moderate inhibitive effect, and the adsorption of RT obeyed the Freundlich adsorption isotherm. Polarization curves showed that RT was a mixed-type inhibitor in phosphoric acid. For the Ur, it had a poor inhibition effect, and acted as a cathodic inhibitor. However, incorporation of RT with Ur improved significantly the inhibition performance. The inhibition efficiency (IE) for RT in combination with Ur was higher than the summation of IE for single RT and single Ur, which was synergism in nature. Polarization studies showed that the RT/Ur mixture acted as a mixed-type inhibitor, which drastically inhibited both anodic and cathodic reactions. The synergistic inhibition effect of RT and Ur could also be evidenced by AFM images. Depending on the results, the synergism mechanism was discussed from the viewpoint of co-adsorption. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Using inhibitors is one of the most practical methods for protection against corrosion, especially in acidic media [1]. Most well-known acid inhibitors are organic compounds containing nitrogen, sulfur, and oxygen atoms. Among them, nitrogencontaining heterocyclic compounds are considered to be the most effective corrosion inhibitors on steel in acid media [2]. The Nheterocyclic compounds of 1,2,3,4-tetrazole derivatives [3–5] and uracil [6] have been used for the corrosion inhibition of iron or steel in HCl and H2 SO4 media. N-heterocyclic compound inhibitors act by adsorption on the metal surface. The adsorption of N-heterocyclic inhibitors takes place through nitrogen heteroatom, as well as those with triple or conjugated double bonds or aromatic rings in their molecular structures. Phosphoric acid (H3 PO4 ) is a medium-strong acid, but it still shows strong corrosiveness on ferrous and ferrous alloys [7]. There is a great need to protect steel materials used in the phosphoric acid industry. Up to now, little work [7–10] appears to have been done on the inhibition of steel in H3 PO4 solutions. Synergistic inhibition effect (synergism) is a combined action of compounds greater in total effect than the sum of the individ-

∗ Corresponding author. Tel.: +86 871 3861218; fax: +86 871 3863150. E-mail address: [email protected] (X. Li). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.02.025

ual effects. Synergistic inhibition is an effective means to improve the inhibitive force of inhibitor, to decrease the amount of usage, to diversify the application of inhibitor. It plays an important role not only in theoretical research on corrosion inhibitors but also in practical work [11]. In previous investigations, the synergistic inhibition effects of organic inhibitor/halide ion mixture [12–16] and organic inhibitor/metallic ion mixture [11,17–20] on corrosion of metal in acidic media have been widely studied. Recently, the synergism of organic inhibitor/organic inhibitor mixture on corrosion of metal in acidic media has also been studied. Villamil et al. [21,22] have studied the synergistic corrosion inhibition of copper by sodium dodecylsufate (SDS) and benzotrizole (BTAH) in 0.5 M H2 SO4 media. Hosseini et al. [23] have investigated the synergism of sodium dodecylbenzenesulphonate (SDBS)/hexamethylenetetramine (HA) mixture on the corrosion of mild steel in 0.5 M H2 SO4 . Tavakoli et al. [24] have studied the synergistic effect on corrosion inhibition of copper by sodium dodecylbenzensesulphonate (SDBS) and 2-mercaptobenzoxazole in 0.5 M H2 SO4 . Qu et al. [25] have studied the synergism between ethylenediamine tetraacetic acid disodium (EDTA) and benzotrizole on the corrosion of steel in 0.1 M HCl. However, higher attentions have been revealed for the synergistic inhibition effect of organic inhibitor/organic inhibitor mixture in HCl and H2 SO4 medium, lower attentions in H3 PO4 medium, especially in a wide concentration range of H3 PO4 solution. In addition, the synergism of two N-heterocyclic compounds mixture on metal corrosion in acidic media is little reported.

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X. Li et al. / Materials Chemistry and Physics 115 (2009) 815–824 Table 1 Relationship between corrosion rate (v) and concentration of RT (c) in 3.0 M H3 PO4 . c (mg l−1 )

Fig. 1. Chemical molecular structures of RT and Ur.

In the present work, the synergism between 1,2,3,4-tetrazole derivatives of red tetrazolium (RT) and pyrimidine derivatives of uracil (Ur) on cold rolled steel in 1.0–10.0 M H3 PO4 solution is first studied by weight loss and polarization methods, and the steel surface is examined by atomic force microscope (AFM). It is expected to get general information on the synergism of N-heterocyclic compounds mixture on the corrosion of steel in phosphoric acid solution. 2. Experimental method

Tests were performed on a cold rolled steel (CRS) of the following composition (given in wt.%): 0.07% C, 0.3% Mn, 0.022% P, 0.010% S, 0.01% Si, 0.030% Al, and bal. Fe. 2.2. Inhibitors Both red tetrazolium (RT) and uracil (Ur) were obtained from Shanghai Chemical Reagent Company of China, and of analytical-reagent (AR) grade. Fig. 1 shows their chemical molecular structures. It is evident that RT contains several nitrogen atoms and Ur contains both nitrogen and oxygen atoms, which could be of a great deal ␲-electrons, and be easily protonated in acidic solution. 2.3. Solutions The aggressive solutions, 1.0–10.0 M H3 PO4 were prepared by dilution of AR grade 85% H3 PO4 with distilled water. The concentration range of inhibitors employed was 25–500 mg l−1 . 2.4. Weight loss and polarization measurements The weight loss and potentiodynamic polarization measurements have been described in detail in our recent reports [26,27]. The immersion time is 6 h, and the experimental temperature is 20–50 ◦ C. All polarization curves were recorded at 20 ◦ C, and the electrode was immersed in tested solution at natural potential for 2 h until a steady state was reached before measurement. The corrosion rate (v) value of weight loss measurement was calculated from the following equation [27]: W St

20 ◦ C

30 ◦ C

40 ◦ C

50 ◦ C

19.95 14.86 13.55 11.63 10.52 9.78 9.45 8.95 8.80 8.43 8.02 7.74 7.32

34.52 29.45 27.11 24.55 21.73 20.28 18.75 17.99 16.86 15.71 14.56 13.91 13.46

73.26 64.48 60.15 56.22 52.98 40.94 46.49 44.32 41.36 39.46 36.72 35.72 34.62

141.79 127.64 120.96 113.28 107.78 103.55 99.91 95.61 92.86 88.78 84.82 80.44 77.01

distilled water, dried with a cold air blaster, and then used for a Japan instrument model SPA-400 SPM Unit atomic force microscope (AFM) examinations.

3. Experimental results and discussion 3.1. Weight loss measurements

2.1. Materials

v=

0 25 50 100 150 200 250 300 350 400 450 475 500

Corrosion rate v (g m−2 h−1 )

3.1.1. Effect of single RT on the corrosion rate The corrosion rate values of CRS with the addition of single RT in 3.0 M H3 PO4 at 20–50 ◦ C are listed in Table 1. It shows that the corrosion rate values (in g m−2 h−1 ) in 3.0 M H3 PO4 solution containing RT decrease as the concentrations of the inhibitor increase, i.e. the corrosion inhibition enhances with the inhibitor concentration. This result may be due to the fact that the adsorption amount and the coverage of inhibitor on the CRS increase with the inhibitor concentration, which makes the CRS surface efficiently separated from the medium [28,29]. Also, Table 1 shows that the corrosion rate of steel increases with increasing temperature both in uninhibited and inhibited solutions. The corrosion rate of steel increases more rapidly with temperature in the absence of inhibitor. 3.1.2. Effect of RT concentration and temperature on inhibition efficiency The values of inhibition efficiencies for different RT concentrations in 3.0 M H3 PO4 are shown in Fig. 2. It shows that the IE increases with the inhibitor concentration ranging from 25 mg l−1 to 500 mg l−1 . The maximum IE was about 63% for RT at the concen-

(1)

where W is the average weight loss of three parallel CRS sheets, S the total area of the specimen, and t is the immersion time (6 h). With the calculated corrosion rate, the inhibition efficiency (IE) was calculated as follows [27]: IE% =

v0 − v × 100 v

(2)

where v0 and v represent the values of the corrosion rate without and with addition of the inhibitor, respectively. IE of potentiodynamic polarization measurement was defined as: IE% =

Icorr − Icorr(inh) Icorr

× 100

(3)

where Icorr and Icorr(inh) are the uninhibited and inhibited corrosion current density values, respectively. 2.5. Atomic force microscope (AFM) Samples of dimension 1.0 cm × 1.0 cm × 0.06 cm were abraded with emery paper (grade 320-500-800) and then washed with distilled water and acetone. After immersion in 3.0 M H3 PO4 without and with addition of 250 mg l−1 RT, 250 mg l−1 Ur and 250 mg l−1 RT + 250 mg l−1 Ur at 20 ◦ C for 6 h, the specimens were cleaned with

Fig. 2. Relationship between inhibition efficiency (IE) and concentration of RT (c) in 3.0 M H3 PO4 .

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Table 2 The linear regression parameters of ln –ln c for RT in 3.0 M H3 PO4 . Temperature (◦ C)

r

Slope

Intercept

n

K (l mg−1 )

Maximum IE (%)

20 30 40 50

0.9927 0.9984 0.9986 0.9972

0.2888 0.4653 0.4851 0.4793

−2.2502 −3.3776 −3.6738 −3.8311

0.2888 0.4653 0.4851 0.4793

0.11 0.034 0.025 0.022

63.3 61.0 52.7 45.7

tration of 500 mg l−1 and 20 ◦ C. The inhibition was estimated to be moderate (45–61%) at 30–50 ◦ C and 500 mg l−1 . The inhibition performances could be explained as follows: Fig. 1 shows that RT can be classified as a 1:1 electrolyte, namely, the organic part (RT+ ) is the cation and the inorganic part (Cl− ) is the anion. It is well known that the steel surface charges positive charge in H3 PO4 because of Ecorr − Eq = 0 (zero charge potential) > 0 [12], so it is difficult for RT+ to approach the positively charged steel surface due to the electrostatic repulsion. On the other hand, since chloride ions (Cl− ) have a smaller degree of hydration, being specifically adsorbed, they create an excess negative charge towards the solution and favor more adsorption of the cations [30], RT+ may adsorb on the negatively charged metal surface (physical adsorption), followed by the partial transference of electrons from the polar atom (N atom) and delocalized ␲-electrons from aromatic ring to the d-orbital of iron atom (chemical adsorption). Fig. 2 also gives that the IE decreases with the experimental temperatures increasing from 20 ◦ C to 50 ◦ C, which indicates that the higher temperatures might cause desorption of the RT from the steel surface. According to Oguzie [31] and Abd El Rehim et al. [32], a decrease in IE with rise in temperature suggests that inhibitor molecules are physically adsorbed on the metal surface, while the reverse behavior suggests chemisorption. Accordingly, we could deduce that the adsorption of RT is mainly the physical adsorption. 3.1.3. Adsorption isotherm of RT on the CRS surface It is generally accepted that organic molecules inhibit corrosion by adsorption at the metal surface and the adsorption depends on the molecule’s chemical composition. Basic information on the adsorption of inhibitor on metals surface can be provided by adsorption isotherm. Attempts were made to fit the experimental data to various isotherms including Frumkin, Langmuir, Temkin, Freundlich, Bockris–Swinkels and Flory–Huggins isotherms. By far the results were best fitted by Freundlich adsorption isotherm equation [33]: ln  = ln K + n ln c

Fig. 3. The relationship between ln  and ln c in 3.0 M H3 PO4 . —20 ◦ C; 䊉—30 ◦ C; —40 ◦ C; —50 ◦ C

3.1.4. Effect of single Ur on inhibition efficiency Fig. 4 shows the influence of concentration of Ur on IE for CRS in 3.0 M H3 PO4 . It can be seen that IE increases with the Ur concentration. According to literature [6], the Ur molecule has four tautomers (Fig. 5). Accordingly, the inhibition efficiency may be explained on the basis of the existence of –OH group in the molecular structure (the tautomeric equilibrium) and two nitrogen atoms. The adsorption of Ur on steel surface may occur by means of the oxygen atoms and also by the delocalization of ␲-electrons of the aromatic ring and the pairs of the nitrogen electrons. However, the maximum IE is only about 30%. In addition, Ur accelerates the corrosion of steel at lower concentration (<100 mg l−1 ), and the minimum IE reaches about −14%. Namely, Ur has a poor inhibition effect on CRS in 3.0 M H3 PO4 .

(4)

where c is the concentration of inhibitor, K the adsorptive equilibrium constant, n the constant describing the nature of steel/media (inhibitor) interface, and  represents the surface coverage calculated by the following relationship [29]: =

(v0 − v)

v0

(5)

From the values of surface coverage, the linear regressions between ln  and ln c were calculated by the computer, and the parameters were listed in Table 2. Fig. 3 is the relationship between ln  and ln c at different temperatures. These results show that all the linear correlation coefficients (r) are almost equal to 1, which indicates the adsorption of RT on steel surface obeys the Freundlich adsorption isotherm. Table 2 shows that 0 < n < 1, which agrees with the general rule [33]. It also indicates that the adsorptive equilibrium constant (K) values decrease with the temperature. Large values of K mean better inhibition efficiency of a given inhibitor. The result is in good agreement with the values of IE obtained from Fig. 2.

Fig. 4. Relationship between inhibition efficiency (IE) and concentration of Ur (c) in 3.0 M H3 PO4 .

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X. Li et al. / Materials Chemistry and Physics 115 (2009) 815–824

Fig. 5. The tautomers of Ur molecule.

3.1.5. Synergistic inhibition effect of RT and Ur Fig. 6 shows the IE in 3.0 M H3 PO4 with different concentration ratios of RT and Ur at a total blend concentration of 500 mg l−1 at different temperatures. It can be seen that the synergistic inhibition effect of RT and Ur is exhibited. Namely, the IE for RT in combination with Ur is higher than the summation of IE for single RT and single Ur, which is synergism in nature. For example, at 20 ◦ C the IEs of 250 mg l−1 RT and 250 mg l−1 Ur are only 52.6% and 17.6%, respectively, while the IE of their mixture is 98.2% at the same conditions, which is more higher than 70.2% (52.6% + 17.6%). High inhibition efficiency (≥90%) was obtained by a mixture of 100–450 mg l−1 RT and 50–400 mg l−1 Ur at 20–30 ◦ C. Fig. 6 also indicates that the IE of RT/Ur mixture decrease with the temperature increasing. To further judge whether synergism is taking place, one has to calculate the synergism parameter (s), as initially proposed by Murakawa et al. [34] for describing the combined inhibition behavior of amines and halide ions. Generally, for the interaction of inhibitors A and B this synergism parameter (s) is defined as follows: s=

1 − IEA − IEB + IEA IEB 1 − IEAB

(6)

where IEA and IEB are the inhibition efficiencies observed with compound A, respectively, B, acting alone, and IEAB is the experimentally observed inhibition efficiency for the mixture A + B (cA and cB in the mixture should be the same as in the corresponding separate situations). The expression actually compares the theoretically expected corrosion rate (numerator), based on the known rates when either A or B are present and on the condition that they do not interact, with the experimentally observed rate in the presence of the inhibitor mixture (denominator). Consequently, in the case where inhibitors A and B have no effect on each other and adsorb at the

Table 3 The synergism parameters of RT and Ur on the corrosion of CRS in 3.0 M H3 PO4 solution at different temperatures. c (RT) (mg l−1 )

25 50 100 150 200 250 300 350 400 450 475

c (Ur) (mg l−1 )

475 450 400 350 300 250 200 150 100 50 25

Synergism parameter (s) 20 ◦ C

30 ◦ C

40 ◦ C

50 ◦ C

2.78 5.31 8.34 9.72 10.64 21.09 16.95 16.26 9.47 8.46 3.31

1.96 3.06 5.36 7.70 8.91 8.70 7.61 7.43 5.43 3.57 2.53

1.17 1.67 3.18 4.96 5.66 5.85 5.61 4.48 3.95 3.29 2.09

1.12 1.59 2.67 3.33 3.93 4.44 4.20 3.90 3.21 2.49 1.50

metal/solution interface independently, s = 1 as in that case the predicted behavior is experimentally confirmed. Alternatively, synergistic effects manifest themselves if s > 1 and antagonistic effects if s < 1. Table 3 shows that the values of s for all investigated concentrations of RT and Ur in 3.0 M H3 PO4 are bigger than the unity, which indicates that there is a true synergism between RT and Ur in 3.0 M H3 PO4 . It should be noted that the values of s are higher than 5.0 for the mixture of 50–450 mg l−1 RT/50–450 mg l−1 Ur at 20 ◦ C and 100–400 mg l−1 RT/100–400 mg l−1 Ur at 30 ◦ C, which can be seemed as the optimized synergistic conditions. From Table 3, it is concluded that the synergism degree at different temperatures follows the general order: 20 ◦ C > 30 ◦ C > 40 ◦ C > 50 ◦ C. Table 4 shows the values of IE for fixing RT concentration at 250 mg l−1 and changing Ur concentration in 3.0 M H3 PO4 . It is found that the complex of RT and Ur has better inhibition efficiency, that is to say, the addition of Ur further increases the IE values comparing with single 250 mg l−1 RT. This suggests that there is a synergistic inhibition effect between RT and Ur. It is should be noted that IE increases with the increasing Ur concentration, and reaches certain value when Ur concentration is at 250 mg l−1 at each experimental temperature, so the more suitable Ur concentration is 250 mg l−1 . Table 4 also indicates that the IE of RT/Ur mixture decreases with the increase of temperature. Table 4 Inhibition efficiencies for 250 mg l−1 RT and different concentrations of Ur in 3.0 M H3 PO4 solution at different temperatures. c (RT) (mg l−1 )

Fig. 6. The relationship between inhibition efficiency and different concentration ratios of RT and Ur at a total blend concentration of 500 mg l−1 in 3.0 M H3 PO4 .

250 250 250 250 250 250 250 250

c (Ur) (mg l−1 )

50 100 150 200 250 300 400 500

IE (%) 20 ◦ C

30 ◦ C

40 ◦ C

50 ◦ C

78.2 88.7 94.6 97.5 98.2 98.3 98.4 98.8

65.7 80.4 90.4 92.1 94.6 94.7 95.7 96.0

54.2 79.2 87.5 89.6 90.1 90.4 91.6 92.3

46.8 57.9 74.1 80.2 86.0 86.7 86.9 87.3

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Table 5 The relationship between corrosion rate (v) and the concentration of H3 PO4 (c) at 20 ◦ C. H3 PO4 concentration, c (M)

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Corrosion rate v (g m−2 h−1 ) Blank

250 mg l−1 Ur

250 mg l−1 RT

250 mg l−1 RT + 250 mg l−1 Ur

8.92 14.42 19.95 22.75 26.63 32.28 41.22 49.92 63.12 81.10

7.02 9.85 16.43 18.01 22.67 34.60 49.22 64.17 77.38 87.42

4.82 7.00 9.45 11.28 13.17 17.58 21.42 26.72 29.02 30.97

0.58 0.98 0.37 1.23 1.33 2.05 3.47 4.42 6.23 9.55

3.1.6. Effect of phosphoric acid concentration on synergistic inhibition effect In order to get general information of the synergism between RT and Ur in a wide concentration range of H3 PO4 , the effect of phosphoric acid concentration on synergism was also studied. Table 5 shows the effect of H3 PO4 concentration (1.0–10.0 M) on the corrosion rate at 20 ◦ C. The corrosion rates obtained both in the absence and presence of inhibitor increase with the H3 PO4 concentration, especially in the range of H3 PO4 concentration from 6.0 M to 10.0 M. In the presence of single 250 mg l−1 Ur, the corrosion rate decreases slightly in 1.0–5.0 M H3 PO4 , while increases in 6.0–10.0 M H3 PO4 comparing with that of blank solution (without inhibitor). However, the corrosion rates decrease in the presence of RT or RT/Ur mixture in the whole H3 PO4 concentration. For example, the corrosion rate is about 9.55 g m−2 h−1 in the presence of 250 mg l−1 RT/250 mg l−1 Ur mixture even when acid concentration is 10.0 M, while the corrosion rate is about 81.10 g m−2 h−1 in the absence of inhibitor. Fig. 7 clearly shows the relationship between IE and H3 PO4 concentration at 20 ◦ C. In the presence of single 250 mg l−1 Ur, IE is poor (14–30%) in 1.0–5.0 M H3 PO4 , and IE is negative (accelerating steel corrosion) in 6.0–10.0 M H3 PO4 solution. For single 250 mg l−1 RT, IE values do not change with the H3 PO4 concentration prominently, and the value is in the range from 45% to 61%, which indicates RT is a moderate inhibitor in the whole studied H3 PO4 solution. The inhibition effect for 250 mg l−1 RT + 250 mg l−1 Ur is good, and the IE value (88–98%) is higher than the summation IE of single Ur and single RT in whole 1.0–10.0 M H3 PO4 . Noticeably, in 6.0–10.0 M H3 PO4 solution, Ur accelerates the steel corrosion, while Ur/RT mixture

performances good inhibition effect. Fig. 7 also shows that IE for RT/Ur mixture does not change with the acid concentration, which would be an advantage in chemical industry. For 7.0 M H3 PO4 level (approximately that reached in dehydrate wet-method process [7]), the IE is 91.6%. For 9.0 M H3 PO4 (corresponding to the concentration of H3 PO4 produced by the hemi hydrate wet-method industrial process [7]) the IE also reaches 90.1%. To study quantificationally the effect of H3 PO4 concentration on synergism, Fig. 8 represents the relationship between synergism parameter (s) and H3 PO4 concentration. Clearly, all s values are higher than unity in 1.0–10.0 M H3 PO4 , which indicates that there is a true synergism between RT and Ur in phosphoric acid. The maximum s obtained from Fig. 8 was 21.09 in 3.0 M H3 PO4 solution.

Fig. 7. Relationship between inhibition efficiency (IE) and the concentration of H3 PO4 (c) at 20 ◦ C.

Fig. 8. Relationship between synergism parameter (s) and the concentration of H3 PO4 (c) at 20 ◦ C.

3.2. Polarization studies 3.2.1. Polarization curves of single RT The polarization behavior of CRS in 3.0 M H3 PO4 in the absence and presence of different concentrations of RT at 20 ◦ C is shown in Fig. 9. It is clear that the inhibitor causes a decrease in the corrosion rate, i.e. shifts the anodic curves to positive potentials and the cathodic curves to negative potentials. This may be ascribed to adsorption of inhibitor over the corroded surface [35]. As the concentration of RT increases, the shift in both anodic and cathodic lines increases. The results could indicate that both cathodic and anodic reactions of CRS electrode corrosion are inhibited by RT in 3.0 M H3 PO4 . The values of corrosion current density (Icorr ), corrosion potential (Ecorr ), cathodic Tafel slope (bc ), and inhibition efficiency (IE)

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Fig. 9. Polarization curves for CRS in 3.0 M H3 PO4 containing different concentrations of RT at 20 ◦ C.

Fig. 10. Polarization curves for CRS in 3.0 M H3 PO4 containing different concentrations of Ur at 20 ◦ C.

Table 6 Potentiodynamic polarization parameters for the corrosion of CRS in 3.0 M H3 PO4 containing different concentrations of RT at 20 ◦ C.

which indicates Ur had poor inhibition effect on the steel corrosion. In the presence of Ur, the slight change of the cathodic Tafel slopes (bc ) indicates that the cathodic process of steel corrosion does not change. From Table 7, the corrosion potential (Ecorr ) shifts negative potential in the presence of Ur, which indicates Ur acts as a cathodictype inhibitor [36].

c (mg l−1 )

Ecorr (mV vs. SCE)

Icorr (␮A cm−2 )

bc (mV dec−1 )

IE (%)

0 25 100 250 500

−447.2 −448.8 −448.7 −447.4 −444.2

1549.0 1090.4 828.8 661.4 514.3

159 158 163 157 158

– 29.6 46.5 57.3 66.8

as functions of RT concentration, were calculated from the curves of Fig. 9 and listed in Table 6. Table 6 reveals that the corrosion current (Icorr ) decreases and IE increases with the inhibitor concentration. The maximum 66.8% IE also indicates that RT has a moderate inhibitive effect. Good agreement between weight loss and polarization curve is obtained. The cathodic Tafel slopes (bc ) do not change remarkably, which indicates that the mechanism of the cathodic corrosion reaction does not change [36]. Furthermore, in the presence of RT, the change of the corrosion potential (Ecorr ) is negligible, therefore, RT can be arranged as a mixed-type inhibitor in H3 PO4 , and its inhibition on CRS is caused by geometric blocking effect [36].

3.2.3. Polarization curves for synergistic inhibition effect of RT and Ur Fig. 11 shows the polarization curves for CRS in 3.0 M H3 PO4 at various concentrations of RT and Ur at a total blend concentration of 500 mg l−1 at 20 ◦ C. Clearly, both anodic and cathodic reactions are drastically inhibited, especially in the presence of the mixtures of 100 mg l−1 RT/400 mg l−1 Ur and 250 mg l−1 RT/250 mg l−1 Ur. The reason may be explained as follows: the co-adsorption of RT and Ur on the CRS surface may occur with the addition of the mixture of RT and Ur. The combined adsorption film of RT and Ur covers both anodic and cathodic reactive sites, which inhibits both anodic and cathodic reactions of steel corrosion. For the RT/Ur mixtures at a total blend concentration of 500 mg l−1 at four different proportion of RT:Ur, Fig. 11 also shows that the degree of inhibiting anodic reaction follows the order: 250 mg l−1 RT/250 mg l−1 Ur mixture > 100 mg l−1 RT/400 mg l−1

3.2.2. Polarization curves of single Ur Both anodic and cathodic polarization curves for CRS in 3.0 M H3 PO4 at various concentrations of Ur are shown in Fig. 10. It can be seen that the cathodic curves shift to negative potentials compared with the blank. It should be noted that the cathodic reaction in the presence of Ur is inhibited, while the anodic reaction of the corrosion process is accelerated. According to Cao [36], the inhibition of Ur is caused by negative electro-catalytic effect. The polarization parameters are listed in Table 7. It can be seen that the corrosion current (Icorr ) decreases and IE increases with Ur concentration. Nevertheless, the maximum IE was only 24.3%, Table 7 Potentiodynamic polarization parameters for the corrosion of CRS in 3.0 M H3 PO4 containing different concentrations of Ur at 20 ◦ C. c (mg l−1 )

Ecorr (mV vs. SCE)

Icorr (␮A cm−2 )

bc (mV dec−1 )

IE (%)

0 25 100 250 500

−447.2 −478.6 −514.4 −502.1 −507.7

1549.0 1542.7 1384.8 1181.9 1172.6

159 156 150 144 142

– 0.4 10.6 23.7 24.3

Fig. 11. Polarization curves for CRS in 3.0 M H3 PO4 containing different concentrations of RT and Ur at 20 ◦ C.

X. Li et al. / Materials Chemistry and Physics 115 (2009) 815–824

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Table 8 Potentiodynamic polarization parameters for the corrosion of CRS in 3.0 M H3 PO4 containing different concentrations of RT and Ur at 20 ◦ C. c (RT) (mg l−1 )

c (Ur) (mg l−1 )

Ecorr (mV vs. SCE)

Icorr (␮A cm−2 )

bc (mV dec−1 )

IE (%)

0 25 100 250 400

0 475 400 250 100

−447.2 −478.1 −445.2 −458.9 −447.2

1549.0 202.9 99.1 63.8 180.1

159 154 119 112 120

– 86.9 93.6 96.0 88.7

Fig. 12. AFM two-dimensional images of CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 ◦ C in 3.0 M H3 PO4 ; (c) after 6 h of immersion at 20 ◦ C in 250 mg l−1 RT + 3.0 M H3 PO4 ; (d) after 6 h of immersion at 20 ◦ C in 250 mg l−1 Ur + 3.0 M H3 PO4 ; (e) after 6 h of immersion at 20 ◦ C in 250 mg l−1 RT + 250 mg l−1 Ur + 3.0 M H3 PO4 .

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Fig. 13. AFM images of CRS surface topography: (a) before immersion; (b) after 6 h of immersion at 20 ◦ C in 3.0 M H3 PO4 ; (c) after 6 h of immersion at 20 ◦ C in 250 mg l−1 RT + 3.0 M H3 PO4 ; (d) after 6 h of immersion at 20 ◦ C in 250 mg l−1 Ur + 3.0 M H3 PO4 ; (e) after 6 h of immersion at 20 ◦ C in 250 mg l−1 RT + 250 mg l−1 Ur + 3.0 M H3 PO4 .

Ur mixture ≈ 400 mg l−1 RT/100 mg l−1 Ur mixture  25 mg l−1 RT/475 mg l−1 Ur mixture. And the degree of inhibiting cathodic reaction follows the different order: 250 mg l−1 RT/250 mg l−1 Ur mixture ≈ 100 mg l−1 RT/400 mg l−1 Ur mixture > 25 mg l−1 RT/475 mg l−1 Ur mixture  400 mg l−1 RT/100 mg l−1 Ur mixture. So it may be concluded that in the presence of the mixture of RT and Ur, both anodic and cathodic processes of steel corrosion were suppressed by the adsorption of RT, while adsorbed Ur along with RT prevent the cathodic process to great extent, which drastically

enhanced the inhibition efficiency and produces strong synergistic inhibition effect. The electrochemical parameters were listed in Table 8. In the mixtures of RT and Ur, the cathodic Tafel slopes (bc ) change to some extent, which indicates that the inhibitor affects the cathodic reactions [35]. The corrosion current (Icorr ) decreases remarkably, which means that both anodic and cathodic reactions are drastically inhibited and have satisfactory IE. For example, the IE calculated from corrosion current density for the complex 250 mg l−1 RT/250 mg l−1

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Ur inhibitor reaches a considerable value (96.0%). The presence of RT/Ur mixture does not remarkably shift the corrosion potential (Ecorr ), so RT/Ur mixture can be arranged as mixed-type inhibitor in H3 PO4 [36]. 3.3. Atomic force microscope (AFM) surface examination The atomic force microscope (AFM) provides a powerful means of characterizing the microstructure [37–40]. The two-dimensional AFM images of CRS surface in 3.0 M H3 PO4 are shown in Fig. 12. Fig. 12(a) shows the CRS surface before immersion seems smooth and shows some abrading scratches. However, it still appears small crevices and is covered with grains, which may be attributed to the defect of steel, and probably an oxide inclusion [20]. As for Fig. 12(b), the CRS surface after immersion in uninhibited 3.0 M H3 PO4 for 6 h was damaged strongly comparing with Fig. 12(a), and covered with the striation-like corrosion products. The corrosion image is different from the corrosion image of steel in HCl [27] and H2 SO4 solution [26]. Fig. 12(c) shows that there is an adsorptive film on the steel surface in the presence of 250 mg l−1 RT, which does not exist in Fig. 12(b). Therefore, it might be concluded that these particles are the adsorption film of the inhibitor, which inhibits the corrosion of CRS [26,27]. Fig. 12(d) also shows that the steel surface in the presence of Ur appears a helix layer image, but the roughness of the surface layer is higher compared to that of the film formed in the presence of RT. In contrast, Fig. 12(e) shows that the steel surface in the presence of both RT and Ur is fully and orderly covered with unique particles, which are quite different from the corrosion product particles (Fig. 12(b)) and do not exist in Fig. 12(c) and (d), and could be ascribed to the co-adsorption film of the mixture of RT and Ur. From Fig. 12(e), it can be seen that that the film is more compact and uniform comparing with Fig. 12(c) and (d); so, it can efficiently protect CRS from corrosion comparing with single RT or Ur. Fig. 13 shows the CRS surface topography. It can be seen from Fig. 13(a) that the micrograph of CRS surface before immersion shows small particles on the surface. Fig. 13(b) shows that the steel surface after immersion in uninhibited 3.0 M H3 PO4 shows the uneven and potholed corrosion products covers layer upon layer. Fig. 13(c) and (d) shows that some relative big particles decorating the steel surface and the inhibitor layer are rough. In the presence of both RT and Ur, Fig. 13(e) shows that the relative smaller and bunched aggregates distribute across the steel surface, and the inhibitor layer becomes much more compact and homogenous. Fig. 14 illustrates the height profiles, which are made along the lines marked in corresponding Fig. 13. The surface roughness of the CRS before immersion is 10.70 nm from Fig. 14(a). Fig. 14(b) indicates that the surface roughness of the CRS after immersion in uninhibited 3.0 M H3 PO4 is up to 578.56 nm, while in the presence of single RT, the roughness decreases to 81.99 nm (Fig. 14(c)). In the presence of single Ur, the roughness is 96.70 nm (Fig. 14(d)), which is higher than that of immersion in the presence of single RT. Fig. 14(e) shows that the surface roughness is only 34.11 nm in the presence of both RT and Ur. In general, the result of the surface roughness agrees with that of Fig. 12. 3.4. Explanation for synergism Incorporation of RT with Ur significantly improved inhibition efficiency (IE), and all the values of synergism parameter (s) are higher than unity, which indicates that there is a true synergistic inhibition effect of RT and Ur in H3 PO4 solution. In order to explain the fact, the following synergism mechanism is proposed: It is well known the steel surface charges positive charge in H3 PO4 [12]. When RT was mixed with Ur in H3 PO4 , Cl− (the anion part of RT) may first adsorb on the positively charged steel surface

Fig. 14. Height profiles of the CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 ◦ C in 3.0 M H3 PO4 ; (c) after 6 h of immersion at 20 ◦ C in 250 mg l−1 RT + 3.0 M H3 PO4 ; (d) after 6 h of immersion at 20 ◦ C in 250 mg l−1 Ur + 3.0 M H3 PO4 ; (e) after 6 h of immersion at 20 ◦ C in 250 mg l−1 RT + 250 mg l−1 Ur + 3.0 M H3 PO4 .

by electrostatic attraction. Then the protonated RT and Ur could easily reach the steel surface, followed by transference of electron from nitrogen hetero-atoms and oxygen atoms to the d-orbital of iron atom (chemical adsorption) at the steel/solution interface. Namely, owing to the co-adsorption of RT and Ur, the tightly protective film was formed on the steel surface. Based on the results of polarization curves of Figs. 9–11, it might be that the anodic process of steel corrosion was suppressed by RT mostly, while the cathodic process was inhibited by the co-adsorption of RT and Ur. Consequently, both anodic and cathodic processes were more inhibited comparing with the case of single RT or single Ur additive, which drastically improves the inhibition efficiency, and produces synergistic inhibition effect. 4. Conclusion 1. RT acts as a moderate inhibitor for the corrosion of CRS in H3 PO4 solution, and the adsorption obeys the Freundlich adsorption isotherm in 3.0 M H3 PO4 . Ur has a poor inhibition effect, and the maximum IE is only about 30%. 2. There is a true synergism between RT and Ur in H3 PO4 solution. Namely, the IE for RT in combination with Ur is higher than the summation of IE for single RT and single Ur. The values of synergism parameter (s) are higher than unity. 3. RT acts as a mixed-type inhibitor in phosphoric acid, while Ur acts as a cathodic inhibitor. The RT/Ur mixture behaves as a

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mixed-type inhibitor, which drastically inhibits both anodic and cathodic reactions. 4. Owing to the co-adsorption of RT and Ur, the denser and compact protective film formed after immersion in the presence of both RT and Ur decreases greatly the steel surface roughness and effectively protects steel from corrosion. Acknowledgement This work was carried out in the frame of research project funded by Key Laboratory for Forest Resources Conservation and Use in the Southwest Mountains of China (Southwest Forestry University) Ministry of Education. References [1] G. Trabanelli, Corrosion 47 (1991) 410. [2] F. Bentiss, M. Traisnel, L. Gengembre, M. Lagrenée, Appl. Surf. Sci. 161 (2002) 194. [3] S. Kertit, B. Hammouti, Appl. Surf. Sci. 93 (1996) 59. [4] P. Morales-Gil, G. Negrón-Silva, M. Romero-Romoa, C. Ángeles-Chávez, M. Palomar-Pardavé, Electrochim. Acta 49 (2004) 4733. [5] H.F. Ma, T. Song, H. Sun, X. Li, Thin Solid Films 516 (2008) 1020. [6] M.Th. Makhlouf, A.S. El-Shahawy, S.A. El-Shatory, Mater. Chem. Phys. 43 (1996) 153. [7] Y. Jianguo, W. Lin, V. Otieno-Alego, D.P. Schweinsberg, Corros. Sci. 37 (1995) 975. [8] E.A. Noor, Corros. Sci. 47 (2005) 33. [9] J.M. Benabdellah, R. Touzani, A. Dafali, B. Hammouti, S. El Kadiri, Mater. Lett. 61 (2007) 1197. [10] M.S. Morad, Corros. Sci. 50 (2008) 436. [11] X.H. Li, S.D. Deng, G.N. Mu, Q. Qu, Mater. Lett. 61 (2007) 2514.

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